Thermal expansion of Cu 6 Sn 5 and (Cu,Ni) 6 Sn 5
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Cu6Sn5 is a common intermetallic compound formed during electrical packaging. It has an allotropic transformation from the low-temperature monoclinic g’-Cu6Sn5 to high-temperature hexagonal g-Cu6Sn5 at equilibrium temperature 186 °C. In this research, the effects of this allotropic transformation and Ni addition on the thermal expansion of g’- and/or g-Cu6Sn5 were characterized using synchrotron x-ray diffraction and dilatometry. A volume expansion during the monoclinic to hexagonal transformation was found. The addition of Ni was found to decrease the undesirable thermal expansion by stabilizing the hexagonal Cu6Sn5 at temperatures below 186 °C and reducing the overall thermal expansion of Cu6Sn5.
I. INTRODUCTION
Cu6Sn5 is an important intermetallic compound (IMC), which commonly forms during interface reactions between most Sn-based solders and Cu substrates.1 The continuous performance demands and minimization of modern electronic products have led to an increased current and accompanying Joule heating.2 As a result, the operating temperature of lead-free solder joints and the volume fraction of IMCs have increased. The thermal expansion of Cu6Sn5 therefore plays an important role in the thermal fatigue of solder joints. Jiang et al.3 investigated the coefficient of thermal expansion (CTE) of Cu6Sn5 between 60 and 160 °C by step heating dilatometry. Wang et al.4 also studied the CTE of Cu6Sn5 formed at the Sn/Cu interface at temperatures from 100 to 250 °C. Zhou et al. suggested a nonlinear thermal expansion and a CTE value higher than the experimental result for monoclinic Cu6Sn5 by first principle computation.5 In the Sn–Cu phase diagram, there is a monoclinic–hexagonal allotropic transformation at a temperature of 186 °C. Both monoclinic and hexagonal Cu6Sn5 can form in a solder joint depending on the alloy composition and soldering conditions. However, the effect of this allotropic transformation on the thermal expansion behavior of Cu6Sn5 has not been considered in previous studies. Ni is an important alloying element in Sn–Cu lead-free solders. The addition of Ni has been associated with such benefits as altered solidification microstructures, an increased volume fraction of the eutectic phase, and a lower propensity for interfacial IMCs to crack during service.6,7 Ni is also actively involved in the interface reaction and has a remarkable influence on the formation and properties of Cu6Sn5.8 It has been reported that an increase in Ni
concentration reduces the enthalpy of Cu6Sn5 and that (Cu,Ni)6Sn5 has a more negative heat of formation.9,10 Ni can also increase the Young’s modulus and hardness of Cu6Sn5.11,12 From a crystallographic point of view, the electron diffraction patterns of monoclinic and hexagonal Cu6Sn5 have been reported.13 Recently, Nogita and Nishimura14,15 found that Ni stabilizes hexagonal Cu6Sn5 at temperatures down to room temperature. The effect of Ni solubility on the thermal expansion behavior of Cu6Sn5, however, remains unknown. In this research, the effects of phase transformation a
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